Mice born with combined lipase deficiency (cld/cld) have very low lipoprotein (LPL) and hepatic (HL) lipase activities, develop severe hypertriglyceridemia, and die within 3 days. The recessive mutation (cld) causing the deficiency is located on chromosome 17, whereas LPL and HL genes are on chromosomes 8 and 11, respectively. Structural genes for the lipases are normal in cld/cld mice. Both lipases are normally synthesized and secreted by parenchymal cells and transferred to endothelial cells where they act. The active secretable form of LPL is thought to be a dimer of identical endo H-resistant glycopeptides. Brown adipocytes cultured from cld/cld mice synthesized and glycosylate lpl but it is endo H- sensitive, inactive and retained in endoplasmic reticulum (ER). Immunocytochemical studies in cultured cells showed that HL, unlike LPL, was secreted by cld/cld hepatocytes, indicating the cld mutation probably affects LPL and HL in different ways. Sucrose gradient centrifugation studies (see below) showed LPL in cld/cld cells is an aggregate of monomers. That Mr of LPL subunits in cld/cld cells with a subnormal number of sugar residues, or trimming of oligosaccharide chains may proceed at least beyond the action of glucosidase I. Whether the cld mutation affects primarily synthesis/processing of oligosaccharides of LPL in ER, transport of the lipase from ER to Golgi, or some other event, is to be resolved. Blocking glucosidase I in ER with CST resulted in synthesis in 3T3-L1 adipocytes of endo H-sensitive lipase which was inactive and not secreted. Sucrose gradient centrifugation studies of LPL extracted from intracellular cisternae of brown adipocytes showed that endo H-sensitive LPL from CST- treated cells was an aggregate of inactive monomers, whereas endo H- sensitive lipase from cells in which Golgi mannosidase I or II had been blocked was dimerized. LPL in the latter cells was also active and secreted. Thus, processing high mannose-type chains to endo H-resistant chains is not required for dimerization, activity of secretion of LPL. The importance of processing LPL to endo H-resistance is not yet known.